1. Field of the Invention
The present invention relates to temperature compensation for a smoke type fire detector.
2. Description of the Related Art
A smoke type fire detector comprises a light emitting element and a light receiving element both lying in a smoke room. Light emanating from the light emitting element is reflected irregularly due to smoke. The irregularly-reflected light is received by the light receiving element. A level of an output signal of the light receiving element is amplified-by an amplifier. A smoke density is then identified using the amplified level of the output signal.
Temperature varies depending on an environment of a site at which a detector is installed. Ambient temperature at the detector varies depending on the installation site. Specifically, the temperature in the vicinity of a roof of a building is very high due to solar thermal power, while the temperature in basement made of a non-adiabatic concrete is very low. There is a great difference in temperature conditions between them. The ambient temperature at the detector is greatly affected by the climate associated with the latitude at an installation site or the presence or absence of an air conditioner.
The sensitivity of a detector is adjusted under substantially the same temperature conditions at a factory. Assuming that the sensitivity of a detector varies depending on temperature, even if the sensitivity is adjusted during a manufacturing process of adjustment at a factory, the sensitivity may vary depending on an installation site.
For example, a light emitting diode (LED) is employed as a light emitting element and a photodiode is employed as a light receiving element. The LED has such a temperature characteristic that the quantity of light emanating therefrom varies at -0.6%/.degree.C. The photodiode has such a temperature characteristic that the output level thereof varies at +0.2%/.degree.C. The overall temperature characteristic of the LED and photodiode comes therefore to -0.6%/.degree.C.+0.2%/.degree.C.=-0.4%/.degree.C. Even when an actual smoke density remains unchanged, if an internal temperature of a smoke type fire detector changes, the output level of the light receiving element varies at -0.4%/.degree.C. Specifically, when the internal temperature of the smoke type fire detector changes by 50.degree. C., the output level of the light receiving element varies by 20%.
Aside from the light emitting element and light receiving element, an amplifier composed of semiconductor elements has a temperature characteristic. As the temperature within a detector changes, the level of an output of the amplifier varies due to the temperature characteristics of the semiconductor elements.
Thus, the output level is affected by a complex temperature characteristic of all components of a detector. The variation in output level is not monotonous relative to a temperature. A conventional method of compensation using a temperature compensation element such as a thermistor cannot therefore achieve temperature compensation satisfactorily.